1. Comparative Primatology and the Evolution of Human Life History
Shannen Robson
Department of Anthropology

2. What is a primate?

3. Primate distribution worldwide
Humans expanded into non-tropical habitats

4. Are humans primates?

5. Or are we too distinct?

6. Behavioral similarities ….

7. Primate phylogeny
reflect shared ancestry
branch lengths reflect time

8. Eg., Trichromatic vision
Bichromatic: blues/greens
Trichromatic: blues/greens & reds

9. Ancestral state: Strepsirhini & Aotus mono- or bi-chromatic
Polymorphic: Platyrrhini (NWM) -- X-linked
Trichromatic: Howling monkeys & all catarrhines

10. Life histories …
“.. lie at the heart of biology; no other field brings you closer to the underlying simplicities that unite and explain the diversity of living things and the complexities of their life cycles”
“LH theory evolution makes the simplifying claim that the phenotype consists of demographic traits connected by constraining relationships, trade-offs”.”
Stearns, 1992

11. Life history traits size at birth growth pattern age at maturity
size at maturity
number, size, &
sex ratio of offspring
reproductive span
length of life

12. Charnov’s Life History Invariant Model

13. Compare great ape life histories
slow
short
equal
Longer lifespan & subadult period = larger body size
Age at last birth neutral
Reproductive rate faster than expected

14. 1. Humans have the slowest life history

15. Are old people a recent novelty?
Oeppen & Vaupel (2002) Science 296:1029

16. Stability of the old/young ratio
Change life expectancy 3-fold & % of senior women in population varies little

17. Survival of hunter-gatherers
N Howell Demography of the
Dobe !Kung (Academic, 1979)
K Hill & AM Hurtado Ache
Life History (Academic, 1996)
NG Blurton Jones et al
AJHB 14, 184 (2002)
e0 = 30 & 33.5% adults >45
e0 = 37 & 39% adults >45
e0 = 33 & 40.4% adults >45 10

18. Compare chimp-human age-specific survival
e0 = 33 & 40.4% adults >45
e0 = 15 & 2.2% adults >45
1. Similar age decline in female fertility
2. Longer adult lifespans, later first birth
3. Much older population age structures

19. Age structure difference part of wider regularity: compare humans & chimps to macaques
0.1
0.2
0.3
0.4
0.5
0.6
0 - 4
5 - 9
Hadza seniors (>45)
Hadza childbearing (15-45)
Hadza juveniles (<15)
Macaca fuscata (Arashiyama West) MSM Pavelka & LM Fedigan AJPA 109, 455 (1999) 26

20. 2. Neutral age at last birth
Human reproductive pattern (Holman 1996)

23. Compare chimp-human age-specific fertility
Births/female/year
Similarity suggests conserved, ancestral pattern

24. How can a female primate increase quantity?
3. Fast reproductive rates
How can a female primate increase quantity?
By increasing her reproductive rate
By increasing her litter size OR

25. Both result in multiple dependent offspring at the same
Both result in multiple dependent offspring at the same time that she must …
… nurse
… carry
… and wean
How can a mother manage?!

26. Usually she can’t Increased quantity compromises quality
For most primates, stacking is unsuccessful
Increased quantity compromises quality

27. Callitrichids & humans increase quantity because they have help
These primates are called ‘cooperative breeders’

28. Callitrichids shorten intervals by …
increasing litter size
postpartum estrus
AND

29. Humans shorten interbirth intervals by …
‘stacking’ sequential dependents
through early weaning
via nonmaternal food-sharing

30. Who helps human moms feed kids?

31. Male hunting? Success is unpredictable Hunter not “owner”
Wives & kids not favored
Must eat everyday!

32. Postmenopausal females - Grandmothers
Gotta eat everyday!
Postmenopausal females - Grandmothers

33. Grandmother hypothesis
1. Chimp-like life history; PlioPleistocene ecology
2. More use of resources difficult for juveniles
3. More maternal provisioning
4. Females with declining fertility help
5. Vigorous perimenopausal females leave more descendants
6. Selection for slower aging, greater longevity
7. Longer adult lifespans; so later maturity, larger size
8. Expand into previously unoccupied habitats